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Article

Removal of Total Phenolic Compounds and Heavy Metal Ions from Olive Mill Wastewater Using Sodium-Activated Jordanian Kaolinite

by
Ethar M. Al-Essa
1,
Khansaa Al-Essa
2,*,
Neda Halalsheh
3,
Abdelmajeed Adam Lagum
1,
Alaa M. Al-Ma’abreh
4,
Hussein Saraireh
1 and
Khaldoun Shatnawi
5
1
Department of Civil Engineering, Faculty of Engineering, Isra University, Amman 11622, Jordan
2
Department of Chemistry, Jerash University, Jerash 26150, Jordan
3
Department of Civil Engineering, Faculty of Engineering, The Hashemite University, Zarqa 13133, Jordan
4
Department of Chemistry, Faculty of Science, Isra University, Amman 11622, Jordan
5
Alltech Middle East FZCO, Showroom No S3B6SR01 JAFZA, Dubai P.O. Box 261444-261444, United Arab Emirates
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(10), 4627; https://doi.org/10.3390/su17104627
Submission received: 26 February 2025 / Revised: 13 May 2025 / Accepted: 15 May 2025 / Published: 18 May 2025
(This article belongs to the Special Issue Development and Optimization of Sustainable Metal Recovery Processes)
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Abstract

Olive mill wastewater (OMW) is deemed a substantial environmental pollutant, particularly in Mediterranean regions. Lower and middle-income countries, including Jordan, suffer from water scarcity and increasing demand for water, especially for drinking and irrigation purposes. Subsequently, the management and treatment of OMW represents a major concern. This study investigates the feasibility of utilizing Jordanian kaolinite as a simple, readily available, green, and sustainable adsorbent to mitigate the environmental impact of untreated or partially treated OMW. In this work, purified kaolinite (PK) was activated with sodium ions at room temperature. The characterization of PK and sodium-activated kaolinite (PK-NaCl) was accomplished using FTIR, XRD, TGA, and BET surface area analyses. The adsorption performance of both PK and PK-NaCl for OMW treatment were evaluated through batch and column experiments. The key physiochemical parameters of OMW were systematically analyzed in all influent and effluent samples to evaluate the treatment efficiency. In all cases, sodium-activated kaolinite significantly enhances treatment efficiency. The adsorption of total phenolic compounds (TPCs) onto both PK and PK-NaCl adsorbents was studied with respect to initial concentration, adsorbent dosage, and temperature. The maximum adsorption capacity was 8.88 mg/g for PK-NaCl, which was higher than that of PK, at an adsorbent dose of 1.0 g and a temperature of 323 K. The Langmuir and Freundlich isotherm models to describe the adsorption equilibrium were implemented, and both displayed good fit with the experimental data. Additionally, the removal efficiencies of heavy metal (i.e., Zn, Fe and Mn) ions were also evaluated. The findings demonstrated that the PK-NaCl completely removed all tested heavy metal ions, regardless of their initial concentrations. Therefore, the cost-effective and easily prepared PK-NaCl significantly improved the adsorption capacity and presents a promising treatment solution for OMW. This approach could be highly beneficial for olive mills across the Mediterranean regions to mitigate the environmental impact of OM waste.

1. Introduction

In regions with a significant olive oil industry, such as Jordan, the improper disposal of olive mill wastewater (OMW) can lead to soil and water pollution, negatively impacting ecosystems owing to its low pH (4.2–6.50), high organic load, the presence of toxic and non-biodegradable total phenolic compounds (TPCs) of polyphenols, and microbial contaminants. Additionally, OMW contains elevated concentrations of inorganic substances, including heavy metal ions, lipids, as well as other contaminants [1,2].
The annual generation of OMW in the Mediterranean basin reached 30 million tons. Particularly, Jordan has approximately 70,000 hectares of olive groves, which contain around 25 million olive trees, producing approximately 220,000 tons of olives annually. This results in a substantial amount of OMW (i.e., estimated at approximately 200,000 m3), which necessitates adequate treatment [3]. The discharge of OMW contributes to environmental pollution, affecting soil, surface water, and groundwater. Using untreated OMW for irrigation or fertilization can result in the bioaccumulation of toxic compounds (especially polyphenols and lipids) in crops. Long-term exposure through the consumption of contaminated crops may pose chronic health risks, including liver and kidney damage, endocrine disruption, potential carcinogenic effects. However, the seasonal production of OMW and the necessity of collecting and transporting it from numerous olive mills to treatment plants can make such systems expensive [4], posing a challenge for small-scale farmers.
Regarding Jordan’s regulations for treated wastewater discharge, the Ministry of Environment has established specific limits for various parameters. For example, the permissible limits for treated effluent intended for restricted irrigation are as follows: pH between 6.0 and 9.0, chemical oxygen demand (COD) ≤ 150 mg/L, total dissolved solids (TDSs) ≤ 1500 mg/L. While there are no explicitly stated limits for total phenolic compounds (TPCs), these compounds are known for their toxicity and are subject to monitoring. In comparison, the European Union’s directives for wastewater discharge are generally more stringent.
For these reasons, OMW treatment and management have become critical concerns. Purified OMW can serve as a valuable resource for fertigation and environmental sustainability [5]. Various treatment methods, such as chemical precipitation [6], solvent extraction [7], biological treatment [8], membrane filtration [9], oxidation processes [10], and adsorption [11], have been explored to reduce TPCs, heavy metal ions, and other contaminants in OMW [12]. Among these methods, adsorption using natural resources has gained significant attention in scientific research as a promising and adaptable technology for sustainable water treatment, owing to its effectiveness in removing a wide range of contaminants [13].
Several adsorbents have been developed, including granular activated carbon [14], zeolites [11], agricultural waste [11], and amberlite [15]. Kaolinite, a naturally occurring and cost-effective clay mineral, is particularly promising for removing TPCs and heavy metal ions from OMW. Jordan has substantial kaolinite reserves, especially in the eastern and southern regions [16,17].
However, kaolinite has a relatively low cation exchange capacity, a limited specific surface area, and the presence of impurities, which restrict its adsorption capacity [18,19]. Therefore, enhancing its specific surface area, developing a more porous structure, and modifying its surface characteristics are essential. Recent studies have focused on activating pure kaolinite to improve its adsorption capacity, making it more suitable for addressing environmental challenges [20,21,22]. These activation processes induce structural and property changes, increasing the surface area and consequently enhancing the adsorption capacity [23,24,25,26]. Some effective activation methods include thermal acid treatment [27], transition metal modification [28], and organic modification [29], all of which enhance adsorption characteristics [30,31].
A sustainable, green, and environmentally friendly approach to kaolinite activation, using safe, affordable, and readily available substances, is highly desirable, particularly for low-income countries such as Jordan. Additionally, addressing the pressing environmental issue of OMW contamination by developing an effective and sustainable adsorption-based treatment method. If left untreated, OMW poses significant environmental and health risks. On the other hand, while most studies focus on the removal of TPC and its derivatives from OMW, minimal attention has been given to the removal of heavy metal ions. Most research has primarily investigated the analysis of Mg, Na, K, and Cu, while less consideration has been given to heavy metals and metalloids removal such as Zn, Fe, Mn, Ni, Cr, and As in OMW compost applications [32]. Thus, the present study aims to evaluate the potential of purified and activated Jordanian kaolinite in treating local OMW. Specifically, the objectives of this research are to activate Jordanian kaolinite with sodium chloride (Na+ ions) and investigate its adsorption performance for removing TPC and selected heavy metal ions (Zn(II), Fe(II), Mn(II)) from OMW. This was achieved using batch and column adsorption techniques with two adsorbents: purified kaolinite (PK) and sodium-activated kaolinite (PK-NaCl). The study also explores the effects of the initial contaminant concentration, adsorbent dosage, and temperature on the efficiency of adsorption processes. Furthermore, the adsorption isotherms were analyzed, and the experimental data were modeled using the Langmuir and Freundlich adsorption models.

2. Materials and Methods

2.1. Materials

The kaolinite clay sample was collected from the airport region of Al Azraq, Jordan (located in eastern Jordan at 31°50′00″ N 36°49′00″ E). The olive mill wastewater (OMW) samples were collected from three different olive mills situated in Jerash city, Jordan (located in northern Jordan at 32°16′12.12″ N 35°53′17.41″ E). All solvents and chemicals were of analytical grade, including sodium chloride (NaCl), silver nitrate (AgNO3) (Puriss), ≥99% (Merck, Darmstadt, Germany), sodium carbonate (Na2CO3) (Acros, Geel, Belgium), Folin–Ciocalteu reagent, and Gallic acid (C7H6O5) (Sigma-Aldrich, Darmstadt, Germany). All glassware was cleaned with twice-distilled water and dried in a hot air oven.

2.2. Raw Kaolinite Purification

The raw kaolinite samples were purified to remove quartz, silica, and any other present impurities using the dispersion-sedimentation method with deionized water [24]. Briefly, the kaolinite sample was ground to a particle size of less than 125 μm using a ball mill. The ground kaolinite was then dispersed in deionized water at room temperature and centrifuged at 700 rpm for 4.0 min. This process was repeated several times. The resulting suspension was heated to 35 °C to evaporate the water, leaving behind the clay fraction, which was subsequently dried in an oven at 60 °C for 24 h. Finally, the purified kaolinite (PK) was crushed, sieved through a 63 μm mesh, and stored in tightly sealed bottles until use in the kaolinite sodium activation experiment.

2.3. Purified Kaolinite Sodium Activation

After the kaolinite purification step, the purified kaolinite was ready for the sodium activation process following the procedure described by [33,34]. Specifically, a suspension containing 17.00 g ± 0.01 g of purified kaolinite and 250 mL of 1.0 mM NaCl was stirred for 48 h at ambient temperature. The obtained suspension was then filtered using a Büchner funnel and washed several times with deionized water to remove any unreacted sodium chloride and other impurities. The presence of chloride ions (Cl−1) was tested against a 5% AgNO3 solution. Finally, the washed sample was dried at 70 °C for 24 h and ground to achieve a particle size of 300 μm. The final activated sample (PK-NaCl) was stored in tightly sealed plastic bottles until use in batch TPC adsorption experiments.
The following reaction occurred:
Ca − kaolinite + 2Na+ ⟶2Na − kaolinite (PK-NaCl) + Ca2+

2.4. Purified and Activated Kaolinite Characterization

The chemical, physical, and thermal properties of PK and PK-NaCl samples were studied using different techniques. Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet NEXUS 670 Spectrophotometer, Thermo Scientific, Waltham, MA, USA) was used to identify functional groups and chemical bonds. X-ray diffraction (XRD, Philips X pert pro diffractometer, PANalytical, Almelo, Netherlands) was performed to examine the crystallographic structure of both kaolinite samples, providing insight into the arrangement of atoms in a crystal lattice. Thermogravimetric analysis (TGA), performed using a NETZSCH STA 409 PG/PC Thermal Analyzer (Selb, Germany), was employed to study the material’s weight changes as a function of temperature and time in a controlled environment. Additionally, Brunauer–Emmett–Teller (BET) surface area analysis (Gemini VII, Micromeritics, Norcross, GA, USA) was used to determine the specific surface area of both PK and PK-NaCl samples.

2.5. Physical-Chemical Characterization of OMW

As reported in a previous study [35], one liter of wastewater samples was centrifuged at 104 rpm for 30 min and filtered twice to obtain a clear and brown solution. The processed wastewater sample was then stored in a well-closed container at −2 °C for further analysis and use in batch experiments. A Crimson PL-700AL meter (Crimson Instruments, Norwalk, CT, USA), was applied to determine the total dissolved solids (TDSs), while a flame photometer (Corning 400, Corning Ltd., Salisbury, UK) was used to determine the sodium (Na+) and potassium (K+) ion concentrations. Moreover, a chemical oxygen demand (COD) Meter (PN HI83099-02 model) and Multiparameter Benchtop Photometer was employed to measure the concentrations of COD, phosphate, nitrate, chlorine, and alkalinity. All analytical methods were performed in triplicate, and statistical analysis was conducted using a completely randomized ANOVA of SAS (2015).

2.6. TPC Concentration in OMW

The TPC concentration was measured using a UV–VIS spectrophotometer (Varian Cary 100) following the Folin–Ciocalteu method, as modified by Leouifoudi and Zyad [36]. Briefly, a mixture of 2.5 mL of 0.2 N Folin–Ciocalteu reagent and 0.5 mL of OMW was prepared and kept in the dark for 5 min. Subsequently, 2 mL of a 75 g/L sodium carbonate solution was added, and the mixture was incubated in the dark for 1 h before being centrifuged at 8000 rpm for 5 min. The absorbance was then measured at 765 nm. Gallic acid was used as a standard for the calibration curve, and the TPC concentration in OMW was expressed as gallic acid equivalents (g GAE/L residue).

2.7. Adsorption Experiments: Batch for TPC Removal, Column for Heavy Metal Ions Removal

Two different sets of batch experiments were conducted to evaluate the efficiency of TPC adsorption, following the procedure described by [34].
The first set of experiments aimed to assess the effect of the adsorbent dosage on TPC removal efficiency. Three different concentrations (0.10, 0.50, and 1.01 g) of both adsorbents (PK and PK-NaCl) were examined using 10 mL of OMW with an initial TPC concentration of 1340.64 mg/L. The experiments were performed at pH = 6 (the value pH of should be less than the pKa of phenol (9.92) and four different temperatures (293, 303, 313, and 323 K) for 4 h [35].
The second set was conducted to investigate the effect of the initial TPC concentration on adsorption capacity. Four different initial TPC concentrations (1215.16, 1340.64, 1442, and 1563.43 mg/L) were tested by adding 1.0 g of PK-NaCl to 10 mL of OMW and stirring the mixture for 4 h at pH = 6 and 303 K.
During the batch adsorption experiments, the adsorbent–OMW mixture was continuously stirred in an Erlenmeyer flask for 4 hr. The solution then underwent filtration using a 0.45 µm microfilter. Finally, the filtration was analyzed using a UV–VIS spectrophotometer (λmax = 765 nm) to determine the TPC concentration. All analytical methods were performed in triplicate.
A third set of experiments was designed using a column technique to explore the influence of the number of portions (ranging from one to ten) of two different adsorbents (PK and PK-NaCl) on the percentage uptake of metal ions, specifically Zn(II), Fe(II), and Mn(II). A 30 cm long glass column with a 20 mm inner diameter was used for the adsorption of Zn (II), Fe (II), and Mn (II) from OMW. The column was packed with 2000 ± 0.1 mg of dried PK and PK-NaCl. A total of 100.0 mL of a 100 ppm metal ion solution (raw OMW without any further modification; pH = 4.6) was divided into ten portions (i.e., 10.0 mL each) and passed through the column at 1.0 mL/min flow rate. The eluent was collected in ten test tubes, and the remaining concentration of the metal ions in each tube was determined using an atomic absorption spectrometer (AAS, Varian Spectra AA–250 Plus, Varian Inc., Palo Alto, CA, USA). The concentration of the adsorbate retained in the adsorbent phase (q, mg/g) and the removal efficiency was calculated according to Equations (2) and (3), respectively [34,37],
q =   ( C i − C e ) m × V  
%   R e m o v a l = ( C i − C e ) C i × 100  
where q is the adsorbent phase concentration at equilibrium (mg adsorbate/g adsorbent), Ci and Ce are the initial and equilibrium TPC concentrations (mg/L), respectively, V is the volume of solution (L), and m is the mass of adsorbent (g).

2.8. Adsorption Isotherm Studies

Adsorption isotherms were studied by fitting the obtained data to the Langmuir and Freundlich models. The adsorption behavior for the removal of different TPC concentrations (1215.16, 1340.64, 1442.00, and 1563.43 mg/L) using a fixed adsorbent dosage (1.0 g of PK and PK-NaCl) were investigated at temperatures of 293, 303, 313, and 323 K over a duration of 4 h.
The linear expression of the Langmuir isotherm model is represented in Equation (4) [38], and the Langmuir dimensionless separation factor (RL) is defined in Equation (5) [39] as follows:
C e q e = 1 K L q m + C e q m    
R L = 1 1 + K L C o    
where qe represents the equilibrium adsorption capacity (mg/g), Ce is the concentration of TPC at equilibrium (mg/L), qm is a maximum adsorption capacity (mg/g), KL is the Langmuir adsorption equilibrium constant (L/mg), and CO is the initial TPC concentration in the solution.
The Freundlich isotherm is expressed by the Equation (6) [40]:
l o g q e = l o g K F + 1 n l o g C e      
where KF represents the adsorption capacity (mg/g) and n is the adsorption intensity, which indicates the favorability of the adsorption process. These constants (KF and n) can be determined by plotting logqe versus logCe.

3. Results and Discussion

3.1. Adsorbents Characterization (PK and PK-NaCl)

3.1.1. FTIR Spectra

In the context of PK and PK-NaCl, FTIR analysis provides insights into changes in the molecular structure due to activation with NaCl. The spectra for two kaolinite samples are shown in Figure 1. The peaks indicate that the absorption band intensities range between 3620.22 and 3697.68 cm−1. These bands represent a typical kaolinite spectrum [41]. Mostly, these bands are ascribed to aluminum–oxygen stretching vibrations [41,42,43]. After sodium activation PK-NaCl, noticeable spectral changes were observed. The characteristic peaks of PK were observed at 3697.65, 3651.58, and 3620 cm−1, corresponding to the stretching vibrations of inner-surface hydroxyl (-OH) groups [33]. The band around 1030 cm−1 is attributed to Si–O stretching vibrations, while the peaks at 912 cm−1 and 798 cm−1 correspond to Al–OH bending and Si–O–Al vibrations, respectively [41,42]. After sodium activation (PK-NaCl), noticeable spectral changes were observed. A reduction in the intensity of hydroxyl (-OH) stretching bands suggests the partial disruption of hydrogen bonding due to sodium intercalation. Additionally, shifts in the Si–O stretching and Al–OH bending vibrations indicate structural reorganization, confirming the successful exchange of Na+ ions with interlayer cations. Moreover, new peaks appeared at 3456.61 cm−1, attributed to the formation of H-O-H stretching absorbed water [41,42,44]. These FTIR results support the hypothesis that sodium activation alters kaolinite’s surface chemistry, enhancing its adsorption capacity for TPC and other heavy metal ions from OMW. The remaining bond positions for the two kaolinite samples and their associated functional groups are presented in Table 1. The peaks observed are as follows: 428–430 cm−1 represents Si-O-Si bending, 537 cm−1 represents Fe-O, Fe2O3 Si-O-Al stretching, 694–695 cm−1 represents Si-O quartz, 798–794 cm−1 represents OH deformation, linked to Al+3, Mg+2, quartz, 912–913 cm−1 represents OH deformation, linked to 2Al+3, 1032 −1110 cm−1 represents Si-O stretching, 1637–1683 cm−1 represents both CO stretching and NH2 bending motions, and 2360 cm−1 represents quartz [41,42,43,44,45,46,47,48].

3.1.2. XRD Spectra

As shown in Figure 2, the typical peaks for kaolinite were confirmed by the XRD patterns at 2θ angles of 12.2°, 20.7°, 24.8°, and 26.5° [49,50,51]. Additionally, the other peaks of 35°, 36°, 38°, 40°, 43°, 46°, 50°, and 55° confirmed the presence of kaolinite.
In the case of PK and PK-NaCl, the XRD results indicate that sodium activation led to changes in peak intensities, a slight increase in intensity at 12.2°, 20.7°, and 24.8°, along with minor shifts due to the intercalation of Na+ ions, which may suggest variations in crystallinity, and partial structural changes [52]. Furthermore, after sodium activation, a slight peak broadening in the range of 35.0° to 40.0° was observed, suggesting increased disorder and improved ion-exchange properties.

3.1.3. Thermogravimetric Analysis (TGA)

The results of the TGA analysis (Figure 3) show that the total mass did not change significantly for both PK and PK-NaCl samples; the temperature increased up to 450 °C. The minor weight loss observed in this range is associated with the removal of physically adsorbed water and possibly some structurally bound water (dehydration process) from the kaolinite structure. As evident, the presence of Na+ ions may slightly alter the binding of water molecules, affecting the dehydration process below 100 °C. Thereafter, the total mass losses for PK were higher than that for PK-NaCl, indicating that the presence of NaCl increases the thermal stability of the microstructure [53].
As the temperature increased from 450 to 700 °C, significant weight loss occurred. This corresponds to a distinct maximum endothermic step, which is associated with the dehydroxylation of kaolinite. This process involves the release of structural hydroxyl (-OH) groups as water vapor [54].
Around 650 °C, PK-NaCl reached its maximum mass loss and began to plateau, whereas for PK, this change occurred at 700 °C. This difference may be attributed to the presence of Na+ ions, which can influence the thermal behavior of kaolinite, potentially shifting the dehydroxylation temperatures. This provides strong evidence that the sodium activation process occurred, potentially modifying the interlayer spacing and affecting dehydroxylation. Above 800 °C, minimal weight loss was detected, indicating the formation of a more thermally stable phase.
Overall, the TGA results confirm that sodium activation affects the thermal behavior of kaolinite, leading to changes in dehydroxylation temperature and structural stability. These modifications could enhance the material’s reactivity and ion-exchange properties, making it more suitable for industrial applications such as adsorption.

3.1.4. BET Surface Area

BET analysis is employed to determine the specific surface area of the adsorbent which incorporates multilayer coverage. This parameter is crucial for understanding the adsorption capacity and efficiency of the adsorbent. A higher surface area typically correlates with more available adsorption sites, which enhances the adsorbent’s ability to capture adsorbates. BET analysis was performed to evaluate the textural properties of PK-NaCl adsorbent and correlate them with its adsorption performance.
The results of the BET surface area are shown in Table 2. These results indicate that the presence of Na+ ions on the adsorbent surface (PK-NaCl) increased the surface area by up to 3.4 times compared to the adsorbent surface area in the absence of Na+ ions (PK). This means that the number of active sites on the surface increased, enhancing adsorption efficiency [55,56]. Kaolinite is a 1:1 clay mineral consisting of a tetrahedral sheet linked to one octahedral sheet. These layers are held together by oxygen atoms, and the interlayer space contains exchangeable cations [57]. The exchangeable cations in the interlayer spaces of kaolinite, such as hydrogen (H+), sodium (Na+), potassium (K+), and calcium (Ca2+) influence the electrostatic forces between the layers [58]. When mono or divalent cations are present, they can partially neutralize the negative charge on the layers, promoting layer separation and swelling. The type, size, and charge of the exchangeable cation significantly affect the swelling behavior of kaolinite. Larger cations, such as Ca2+, impede the expansion of the interlayer space, reducing swelling [59]. Additionally, cations with higher charges, such as aluminum (Al3+), have a stronger interaction with the negatively charged kaolinite layers and can influence swelling [60]. Monovalent cations like sodium (Na+) are generally more effective at promoting swelling, as they can easily migrate and hydrate between the layers, creating separation and allowing for greater interlayer expansion [61].
The overall results of the characterization of sodium-activated kaolinite (FTIR, XRD, TGA and BET analyses) are consistent and confirm that the sodium activation process has taken place.

3.2. Results of Physical-Chemical Characterization of OMW

The highest concentration of mineral salts in untreated OMW (i.e., control sample) was potassium (6366.3 mg/L), highlighting its role as a key mineral element in olives. Additionally, a notable concentration of phosphates (4120 mg/L) was observed in the untreated OMW. Other detected concentrations included sodium (297.9 mg/L), alkalinity (2000 mg/L), nitrate (360 mg/L), and total chlorine (20 mg/L). The presence of a significant amount of polyphenolic compounds (i.e., natural antioxidants found in olives) was indicated by an average concentration of 1.34 g GAE/L. Additionally, the untreated OMW exhibited a high chemical oxygen demand (COD) value, averaging 12,000 mg/L, reflecting a substantial organic compounds load. Similarly, the elevated total dissolved solids (TDSs) concentration of 34,700 mg/L suggests the considerable presence of dissolved substances, including inorganic salts, minerals, and other dissolved compounds. Overall, the measured parameter concentrations align with literature values and fall within expected ranges, as shown in Table 3. The physicochemical analysis of OMW is presented as the mean ± standard deviation of triplicate measurements as shown in Figure 4. The concentration of each parameter was statistically evaluated using a completely randomized ANOVA of SAS (2015). These contrasts were formed to compare the main effects of sodium-activated kaolinite (PK vs. PK-NaCl). Additionally, the p-value was determined and compared against a significance threshold of p < 0.05 value.
The results show the removal efficiency percentage by comparing untreated OMW with OMW treated using PK, and PK-NaCl. Both kaolinite samples demonstrated significant adsorption capability; however, PK-NaCl significantly enhanced the removal efficiency. For example, 40.3% of TPC was removed using PK-NaCl compared to untreated OMW, while PK alone removed only 6.5% of TPC. Similarly, 37.5% of alkalinity was removed using PK-NaCl compared to untreated OMW, whereas PK alone removed 7.5% of alkalinity. These results demonstrate that PK-NaCl improved the TPC removal by 33.6% (p < 0.01) and alkalinity removal by 30% (p < 0.01) as compared to PK. Likewise, the removal % of TDS, nitrate, potassium, total chlorine, and sodium increased significantly by 20.8% (p < 0.001), 15.3% (p < 0.001), 11.4% (p < 0.01), 10% (p < 0.01), and 8.4% (p < 0.05), respectively, when using PK-NaCl instead of PK. Conversely, PK-NaCl did not show a significant difference (p > 0.05) in removing phosphate (79.2% vs. 76.9%) and COD (91.9% vs. 91.5%) compared to PK.
The limited effectiveness of PK-NaCl compared to PK in removing phosphate and COD can be attributed to the nature of the activating agent and the surface chemistry of the resulting material. NaCl activation primarily induces physical changes and does not significantly enhance the presence of functional groups or metal oxides (such as Fe, Al, or Ca) that are essential for phosphate binding through ligand exchange or precipitation. Unlike multivalent metal ions, Na+ is monovalent and does not form insoluble complexes with phosphate, thus offering limited potential for chemical adsorption or precipitation. For COD removal, the NaCl-activated material may contain fewer oxygen-containing functional groups. As a result, there is no significant difference between the interaction of PK-NaCl and PK with organic matter.
These findings confirm that PK-NaCl significantly improved the removal efficiency of most OMW parameters compared to PK adsorbent. Additionally, these results indicate that Na+ ions play a role in kaolinite activation by enhancing its overall adsorption performance.

3.3. TPC Removal by Batch Adsorption Experiments

3.3.1. Effect of Adsorbent Dosage

The overall results of Figure 5 show that the efficiency of TPC removal using PK-NaCl was higher compared to PK at all temperatures and adsorbent dosages. This indicates that PK is effectively activated by Na+ ions. Therefore, to reduce the required PK dosage, it is necessary to activate it into superior PK-NaCl. On the other hand, there is a clear dosage-dependent relationship between the amount of PK-NaCl used and the percentage of TPC removal. At a constant TPC concentration and volume, as the dosage of PK-NaCl increased, the efficiency of TPC removal also increased. This is due to the greater availability of unsaturated adsorption active sites, along with an increase in surface area. Consequently, the adsorption capacity of PK-NaCl improves, meaning that the efficiency of TPC removal (%) is a function of its mass.
Moreover, increasing the temperature positively prompted the efficiency of TPC removal, indicating that the adsorption process is endothermic. As the solution temperature rises, TPC molecules gain more kinetic energy, increasing their mobility and allowing them to diffuse faster from the bulk phase to the solid phase. Furthermore, higher temperatures are proposed to increase the number of surface adsorption active sites due to the dissociation of certain surface components onto PK-NaCl [65]. Additionally, temperature has a notable effect on sodium-activated kaolinite, whereby Na+ ions present in the diffusion layer tend to migrate toward and accumulate on the kaolinite surface, increasing the Na+ ions content on the kaolinite surface. Therefore, the reaction feasibility between Na+ and Ca2+ improves [66].

3.3.2. Effect of Initial TPC Concentration

The effect of the initial concentration of TPC on the adsorption capacity of PK and PK-NaCl at different temperatures of (293, 303, 313, and 323 K) is shown in Figure 6.
First, it is apparent that, as the concentration of TPC increased, the adsorption capacity also increased. This positive correlation is common in adsorption processes, where higher initial concentrations result in more TPC being available in the solution for binding to the active sites of adsorbent. Indeed, the initial TPC concentration provides a crucial driving force to overcome the mass transfer resistance. A similar phenomenon has been observed in the adsorption using several of adsorbents [67,68,69,70,71].
In addition, the adsorption capacity of TPC using PK-NaCl is higher than the PK, as shown in Figure 6. This can be attributed to the larger specific surface area with more active sites of PK-NaCl. Moreover, NaCl dissociates into Na+ and Cl− ions in an aqueous solution, creating a strong electrostatic field around the cations and anions. Consequently, an oriented array of water molecules is formed around these ions, enhancing the interaction between water molecules and phenolic compounds. The Na+ ions in PK-NaCl play an essential crucial role in enhancing the adsorption capacity through the electrostatic attraction [72].
The presence of NaCl crystals also contributes to increased permeability, leading to a higher cation exchange capacity [73]. Additionally, it reduces interparticle repulsion among negatively charged plates [74]. The possible mechanism of TPC adsorption onto PK-NaCl likely involves the ionic interactions between phenolate ions and Na+ ions on the surface of the prepared adsorbent (PK-NaCl), resulting in the formation of sodium phenoxide (C6H5ONa) [34]. Furthermore, across all temperatures, PK-NaCl proved to be more effective in adsorbing TPC compared to PK. As well, as the temperature increased, the adsorption capacity also increased. This indicates that the adsorption process is temperature-dependent, with higher temperatures facilitating greater adsorption capacities for TPC using PK-NaCl. The increase in adsorption capacity with temperature is a common phenomenon in adsorption processes and can be attributed to the enhanced molecular mobility at higher temperatures, which increases the likelihood of TPC molecules being adsorbed onto the surface of the adsorbent [40,75,76].

3.4. Adsorption Isotherm Studies

The Langmuir and Freundlich isotherm plots for TPC adsorption on both PK and PK-NaCl are shown in Figure S1a–d (see Supplementary Information) at a fixed adsorbent dose of 1.0 g, with an initial concentration of 1340.64 mg/L, and at different temperatures of 293, 303, 313, and 323 K.
As shown in these figures, both isotherm plots exhibit excellent linearity (R2 > 0.90). However, the linearized Langmuir isotherm plots have slightly better correlation coefficients (R2) than the Freundlich plots, indicating that the adsorption process fitted both models.
The Langmuir isotherm describes monolayer homogeneous adsorption on a surface containing a finite number of binding sites, with uniform energies and negligible interactions between adsorbed molecules [77].
From the Langmuir constants, the values of RL indicate the type of isotherm: unfavorable adsorption (RL > 1), linear adsorption (RL = 1), favorable adsorption (0 < RL < 1), or irreversible sorption (RL = 0) [78]. As shown in Table 4, the RL values for TPC adsorption onto both PK and PK-NaCl fall within the range 0 < RL < 1, indicating a favorable sorption process. Moreover, the values of qm, which are related to adsorption capacity, indicate that increasing the temperature leads to an increase in adsorption capacities. Additionally, the qm values for PK-NaCl are greater than those for PK, providing strong evidence that all the obtained results are consistent.
The Freundlich isotherm parameters for TPC adsorption on both PK and PK-NaCl are shown in Table 4. The adsorption coefficients (KF, n) align well with the conditions supporting favorable adsorption, where higher KF values indicate better adsorption. Notably, KF values increase with increasing temperature. In addition, all n values are greater than one, and since they exceed 4, this indicates that the adsorption is highly favorable [79]. Furthermore, n values for PK-NaCl are greater than those for PK and also increase with rising temperature. As a result, the overall obtained data are coherent, reproducible, and reliable.
The adsorption capacity of TPC using PK-NaCl, as investigated in this study, is compared with various other natural adsorbents reported in the literature (Table 5). The findings suggest that PK-NaCl exhibits a competitive adsorption capacity, making it a potential candidate for treating TPC-contaminated water.

3.5. Heavy Metals Removal by Column Adsorption Experiments

The percent uptake for metal ions of (Zn(II), Fe(II), Mn(II)) adsorbed onto both PK and PK-NaCl using the column technique, is presented in Figure 7. The overall results indicate a higher affinity, adsorption capacity, and percentage uptake of all metal ions for the PK-NaCl adsorbent compared to PK. This suggest a greater density of available adsorption sites in PK-NaCl, as confirmed by the findings. These results align with those obtained from batch experiments. Moreover, the adsorption of the metal ions by both adsorbents follows the order: Mn(II) > Fe(II) > Zn(II). While the percentage uptake of all metal ions is significant, it is evident that PK-NaCl can completely remove all heavy metal ions from OMW, regardless of their initial concentrations. Therefore, it is highly recommended that these two low-cost and readily available adsorbents are used for treating OMW contaminated with heavy metal ions, as well as other types of industrial wastewater.
The observed adsorption order of metal ions can be explained by their ionic radii. The ionic radius follows the sequence: Mn (II) > Fe (II) > Zn (II). If metal adsorption onto kaolinite occurs electrostatically, ions with a larger ionic radius should be adsorbed more strongly [84]. However, the selectivity sequence observed for adsorption is Mn (II) > Fe (II) > Zn (II). Another influencing factor is hydration energy. Hydration energy decreases as the ionic radius increases, and increases as the hydration shell expands [85]. Metals with higher hydration energy tend to remain in the solution phase. When forming an aqua complex [M(H2O)m]n+, where m is larger than six, the metal ion is surrounded by m water molecules in a structured hydration shell that differs from the rest of the surrounding water. A metal ion with a smaller radius has a stronger hydration shell compared to a metal ion with a larger radius. Therefore, ions with larger radii exhibit higher absorptivity than those with smaller radii [86]. Among the three cations studied, Mn(II) has the lowest hydration energy, making it the most likely to lose water ligands and become a bare Mn(II) ion when adsorbed onto both adsorbents. Additionally, OMW generated at Jerash City is characterized by high concentrations of Zn(II), Fe(II), and Mn(II), at 2025, 1347, and 1242 ppm, respectively. Figure 8 summarizes the high adsorption capacities (mg/g) of each adsorbent, demonstrating their effectiveness in heavy metal ions removal.
The data of Figure 8 represent the initial concentrations of heavy metal ions, Fe(II), Zn(II), and Mn(II) and their respective adsorption capacities q (mg/g) using both PK and PK-NaCl adsorbents. When using the PK, the adsorption capacities were 1999.99 mg/g for Fe(II) (initial concentration: 2025 ppm), 1290.62 mg/g for Zn(II) (initial concentration: 1341 ppm), and 1240.50 mg/g for Mn(II) (initial concentration: 1242 ppm).
These results indicate that nearly all Fe(II), Zn(II), and Mn(II) ions were effectively adsorbed by the PK adsorbent. In the presence of Na+ ions, the adsorption capacities slightly increased to 2017.76 mg/g for Fe(II), 1310.99 mg/g for Zn(II), and 1241.48 mg/g for Mn(II). This confirms that the addition of Na+ ions enhances the adsorption process, as previously discussed.
The % uptake of Fe(II), Zn(II), and Mn(II) ions using PK-NaCl, as investigated in this study, is compared with various other natural adsorbents reported in the literature (Table 6). The results demonstrate that the % uptake of heavy metal ions is exceptional relative to other adsorbents, suggesting that PK-NaCl is a promising, consistent, reliable, and effective material for treating water contaminated with heavy metal ions.

4. Conclusions

The adequate treatment and management of OMW discharge is essential, particularly in Jordan. The high levels of toxic TPC and heavy metal ions, combined with the significant production of OMW and a critical shortage of water resources, pose a serious environmental challenge that must be addressed. A sustainable, cost-effective, and environmentally friendly method is needed to remove TPC and heavy metal ions from OMW.
Using natural sodium-activated Jordanian kaolinite has shown promising results, with a significant 33.6% increase in adsorption capacity, reaching qm (8.88 mg/g) at an optimal dose of 1 g at 323 K. Equilibrium adsorption closely followed both the Langmuir and Freundlich models. Both kaolinite samples, PK and PK-NaCl, demonstrated significant adsorption capabilities; however, PK-NaCl notably enhanced the removal efficiency compared to PK. The physicochemical properties of OMW improved significantly after treatment. For instance, PK-NaCl removed 40.3% of TPC, while PK removed only 6.5%. Similarly, PK-NaCl reduced alkalinity by 37.5%, whereas PK achieved only a 7.5% reduction. Additionally, the removal efficiency of Zn(II), Fe(II), and Mn(II) ions exceeded 95% for both PK and PK-NaCl adsorbents.
These findings confirm that PK-NaCl significantly improves the removal of most OMW contaminants compared to PK. Furthermore, the sodium activation process is simple, practical, feasible, and cost-effective. The results also indicate that Na+ ions play a crucial role in kaolinite activation by enhancing its surface properties, improving overall adsorption performance in removing TPC and heavy metal ions. The results of this study contribute to the advancement of wastewater treatment technologies by offering an alternative adsorbent that can be applied in industrial and environmental remediation efforts.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su17104627/s1. Figure S1: (a) Langmuir isotherm of PK adsorbent of TPC; (b) Freundlich isotherm of PK adsorbent of TPC; (c) Langmuir isotherm of PK-NaCl adsorbent of TPC; (d) Freundlich isotherm of PK-NaCl adsorbent of TPC.

Author Contributions

Conceptualization, K.A.-E. and E.M.A.-E.; methodology, K.A.-E.; software, K.S.; validation, N.H., K.A.-E. and H.S.; formal analysis, A.M.A.-M.; investigation A.A.L.; resources, A.A.L.; data curation, K.A.-E., A.A.L.; writing—original draft preparation, K.A.-E.; writing—review and editing, K.A.-E. and E.M.A.-E.; visualization, E.M.A.-E.; supervision, K.A.-E.; project administration, K.A.-E.; funding acquisition, K.A.-E. and E.M.A.-E. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully acknowledge the support of Jerash University and Isra University. Special thanks are extended to Khaldoun Shatnawi for his valuable help with the statistical analysis.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.

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Figure 1. FTIR spectra of PK and PK-NaCl adsorbents.
Figure 1. FTIR spectra of PK and PK-NaCl adsorbents.
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Figure 2. The results of XRD analysis of PK and PK-NaCl adsorbents.
Figure 2. The results of XRD analysis of PK and PK-NaCl adsorbents.
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Figure 3. TGA curves of PK and PK-NaCl adsorbents.
Figure 3. TGA curves of PK and PK-NaCl adsorbents.
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Figure 4. Characterization parameters of OMW samples. The columns ■, ■, and ■ represent the untreated OMW (i.e., control sample), OMW treated with PK, and OMW treated with PK-NaCl, respectively. The red arrow → represents the removal efficiency percentage as compared to the untreated sample. Error bars represent the standard deviation of triplicate experiments. a, b, and c mean that values in the same column with different superscripts are significantly different (p < 0.05).
Figure 4. Characterization parameters of OMW samples. The columns ■, ■, and ■ represent the untreated OMW (i.e., control sample), OMW treated with PK, and OMW treated with PK-NaCl, respectively. The red arrow → represents the removal efficiency percentage as compared to the untreated sample. Error bars represent the standard deviation of triplicate experiments. a, b, and c mean that values in the same column with different superscripts are significantly different (p < 0.05).
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Figure 5. The effect of the adsorbent dosage (mass (g)) on the efficiency of TPC removal at different temperatures T (K). Initial TPC Conc (mg/L) = 1340.64, contact time = 4 h, OMW sample volume (L) = 0.01 and, pH = 6.
Figure 5. The effect of the adsorbent dosage (mass (g)) on the efficiency of TPC removal at different temperatures T (K). Initial TPC Conc (mg/L) = 1340.64, contact time = 4 h, OMW sample volume (L) = 0.01 and, pH = 6.
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Figure 6. Effect of initial TPC concentration on the adsorption capacity. Operating conditions: 1.0 g of adsorbents, 10 mL of OMW for 4 h at 293, 303, 313 and 323 K. The x axis (Ce) represents the concentration of TPC at equilibrium (mg/L), and the y axis (qe) represents the equilibrium adsorption capacity (mg/g).
Figure 6. Effect of initial TPC concentration on the adsorption capacity. Operating conditions: 1.0 g of adsorbents, 10 mL of OMW for 4 h at 293, 303, 313 and 323 K. The x axis (Ce) represents the concentration of TPC at equilibrium (mg/L), and the y axis (qe) represents the equilibrium adsorption capacity (mg/g).
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Figure 7. The effect of several portions on the percentage of uptake metal ions.
Figure 7. The effect of several portions on the percentage of uptake metal ions.
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Figure 8. Heavy metal (II) analysis where ■, ■, and ■ represent initial metal ion concentration (mg/L), q (mg/g) using PK-NaCl, and q (mg/g) using PK, respectively.
Figure 8. Heavy metal (II) analysis where ■, ■, and ■ represent initial metal ion concentration (mg/L), q (mg/g) using PK-NaCl, and q (mg/g) using PK, respectively.
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Table 1. The bond positions (wavenumber, cm−1) for PK and PK-NaCl adsorbents, and the related functional groups.
Table 1. The bond positions (wavenumber, cm−1) for PK and PK-NaCl adsorbents, and the related functional groups.
Wavenumber (cm−1)Functional GroupsKaolinite AdsorbentsReferences
2360QuartzPK[45]
1637–1683Both CO stretching and NH2 bending motionsPK and PK-NaCl[42]
1032–1110Si-O stretchingPK and PK-NaCl[46]
912–913OH deformation, linked to 2Al+3PK and PK-NaCl[43]
798–794OH deformation, linked to Al+3, Mg+2, quartzPK and PK-NaCl[47]
694–695Si-O quartzPK and PK-NaCl[48]
537Fe-O, Fe2O3 Si-O-Al stretchingPK[44]
428–430Si-O-Si bendingPK and PK-NaCl[41]
Table 2. BET surface area for PK and PK-NaCl adsorbents.
Table 2. BET surface area for PK and PK-NaCl adsorbents.
Adsorbent TypeSurface Area (m2/g)
PK38.3
PK-NaCl131.1
Table 3. The characterization of untreated OMW compared to literature value ranges [35,62,63,64].
Table 3. The characterization of untreated OMW compared to literature value ranges [35,62,63,64].
ParametersOMW AnalysisLiterature
Ranges Values
Potassium, mg/L6366.3639–10,800
Phosphate, mg/L412031.8–1820
Sodium, mg/L297.9200–570
Alkalinity, mg/L20003150–9070
Nitrate, mg/L360350–390
Total Chlorine, mg/L2033.3–142.7
TPC, g GAE/L1.340.26–10.7
COD, mg/L12,0001900–220,000
TDS, mg/L34,7005900–103,200
Table 4. Values of Langmuir and Freundlich isotherm constants for the adsorption of TPC onto both adsorbents.
Table 4. Values of Langmuir and Freundlich isotherm constants for the adsorption of TPC onto both adsorbents.
Isotherm
Model		AdsorbentIsotherm ConstantTemperature (K)
293303313323
Langmuir IsothermPK-NaClRL 0.12820.11230.09760.0754
PK0.12200.10670.09690.0750
PK-NaClKL0.00510.00590.00690.0090
PK0.00540.00620.00700.0091
PK-NaClqm7.81688.02018.06408.8828
PK7.75037.95758.02628.8121
Freundlich IsothermPK-NaCln4.95675.41515.96806.7648
PK4.84815.29585.94046.7248
PK-NaClKF1.62901.92712.22222.8905
PK1.57741.87102.21672.8059
Table 5. Comparison of the TPC adsorption capacities of various natural adsorbents.
Table 5. Comparison of the TPC adsorption capacities of various natural adsorbents.
Natural AdsorbentAdsorption Capacity
q (mg/g)		Reference
Chitosan6.00[80]
Clay-solidified grouting curtain8.4[81]
Natural soil N12.67[82]
Natural soil N32.49[82]
Natural soil R1.66[82]
Bentonite0.25[83]
Kaolinite0.47[83]
HDTMA-kaolinite2.35[83]
PTMA-kaolinite0.68[83]
PK-NaCl8.88Present study
Table 6. Comparison of % uptake of different heavy metal ions of various natural adsorbents.
Table 6. Comparison of % uptake of different heavy metal ions of various natural adsorbents.
Adsorbent (Heavy Metal Ion)% UptakeReference
RHAC4 (Zn(II))84.6[87]
RHAC6 (Zn(II))86.0[87]
Ash-RH (rice husk ash) (Fe(II))72.0[88]
Bentonite (Fe(II))98.2[89]
Zeolite (Fe(II))96.1[89]
FHA-Kaolinite (Zn(II))19.0[24]
FHA-Kaolinite (Cd(II))52.0[24]
FHA-Kaolinite (Pb(II))55.0[24]
KTD-Kaolinite (Zn(II))6.0[24]
KTD-Kaolinite (Cd(II))28.0[24]
KTD-Kaolinite (Pb(II))59.0[24]
ACC (Pb(II))96.12[90]
ACC (Cd(II))80.11[90]
Oak-AC (Pb(II))98[91]
AC-C (Cd(II))76.45[92]
PK-NaCl (Zn(II))99.9Present Study
PK-NaCl (Mn(II))99.9Present Study
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Al-Essa, E.M.; Al-Essa, K.; Halalsheh, N.; Lagum, A.A.; Al-Ma’abreh, A.M.; Saraireh, H.; Shatnawi, K. Removal of Total Phenolic Compounds and Heavy Metal Ions from Olive Mill Wastewater Using Sodium-Activated Jordanian Kaolinite. Sustainability 2025, 17, 4627. https://doi.org/10.3390/su17104627

AMA Style

Al-Essa EM, Al-Essa K, Halalsheh N, Lagum AA, Al-Ma’abreh AM, Saraireh H, Shatnawi K. Removal of Total Phenolic Compounds and Heavy Metal Ions from Olive Mill Wastewater Using Sodium-Activated Jordanian Kaolinite. Sustainability. 2025; 17(10):4627. https://doi.org/10.3390/su17104627

Chicago/Turabian Style

Al-Essa, Ethar M., Khansaa Al-Essa, Neda Halalsheh, Abdelmajeed Adam Lagum, Alaa M. Al-Ma’abreh, Hussein Saraireh, and Khaldoun Shatnawi. 2025. "Removal of Total Phenolic Compounds and Heavy Metal Ions from Olive Mill Wastewater Using Sodium-Activated Jordanian Kaolinite" Sustainability 17, no. 10: 4627. https://doi.org/10.3390/su17104627

APA Style

Al-Essa, E. M., Al-Essa, K., Halalsheh, N., Lagum, A. A., Al-Ma’abreh, A. M., Saraireh, H., & Shatnawi, K. (2025). Removal of Total Phenolic Compounds and Heavy Metal Ions from Olive Mill Wastewater Using Sodium-Activated Jordanian Kaolinite. Sustainability, 17(10), 4627. https://doi.org/10.3390/su17104627

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